In recent years, radio transmission devices with increasingly high performance have been realized, but devices with even higher performance, able to be adapted to a plurality of radio transmission systems, are needed. An example of this type of device would be one incorporating the functions of both (1) a PDC (Personal Digital Cellular: the so-called regular portable phone) device, which has a large transmission area and enables transmission even when moving at high speed; and (2) a PHS (Personal Handy-phone System, or the so-called "Second-Generation Cordless Telephone System") device, with its low telephone charges and high-speed data transfer; thereby enabling switching between these functions as needed.
A terminal device for a portable phone able to function as a shared PDC/PHS unit could be realized, for example, by a terminal device 31 shown in FIG. 25. Audio signals picked up by a microphone 32 are sent through an amplifier 33 to an analog/digital converter 34, where they are converted to digital signals, which are sent to a processing circuit 35, where they are modulated into transmission signals. Received signals, on the other hand, are demodulated by the processing circuit 35, converted into analog signals by a digital/analog converter 36, and then amplified by an amplifier 37 and turned into sounds by a speaker 38.
An input operating means 40, such as a ten-key pad, and a display means 41, realized by a liquid crystal panel or other device, are connected to the processing circuit 35 through an interface 39.
The transmission signals from the processing circuit 35, after amplification by an amplifier al, are sent through either of two filters fcl or fsl, and transmitted from an antenna 42. The received signals received by the antenna 42, on the other hand, are sent through either of two filters fc2 or fs2 to an amplifier a2, where they are amplified, and then sent to the processing circuit 35. The filters fc1 and fc2 are PDC band pass filters with center frequency set in the vicinity of 1.5 GHz, while the filters fs1 and fs2 are PHS band pass filters with center frequency set in the vicinity of 1.9 GHz.
In order to switch between the pair of filters fc1, fc2 and the pair of filters fs1, fs2 when switching from PDC to PHS use or vice versa, the terminal device 31 is provided with two pairs of switches (s11 and s12; s21 and s22) and a control circuit 43 which performs the switching control. The control circuit 43 performs switching control by operating the switches s11 and s12 or s21 and s22 in concert according to whether the terminal device 31 is being used with the PDC or PHS system, and whether the transmission or reception time slot is in effect.
It can be seen from the explanation above that the terminal device 31 could be greatly reduced in size if filter characteristics were made variable.
In order to achieve variable filter characteristics in a high-frequency filter for radio transmission devices, conventional art has often used a variable-capacity diode, as disclosed, for example, by Japanese Unexamined Patent Publication Nos. 7-131367/1995, 61-227414/1986, 5-63487/1993, 5-235609/1993, 7-283603/1995, and 8-102636/1996.
As one example, FIG. 26 shows the equivalent circuit of a voltage-controlled variable-passband filter 1 according to Japanese Unexamined Patent Publication No. 7-131367/1995. As is evident from the voltage-controlled variable-passband filter 1, the conventional art is structured so that variable-capacity diodes 4 and 5 are connected between input/output terminals p1 and p2 in a filter circuit having resonator patterns 2 and 3, thereby ensuring that desired filter characteristics are obtained by changing the capacitance of the variable-capacity diodes 4 and 5 by means of a direct-current control voltage applied to a control terminal p3.
Another example is a resonating circuit for use in oscillating circuits and elsewhere, such as that disclosed by Japanese Unexamined Patent Publication No. 59-229914/1984. As shown in FIG. 27, in resonating circuit 11 a plurality of series variable-capacity diodes 12 and a plurality of series variable-capacity diodes 13 are connected in reverse series with relation to each other, and a coil 14 is connected in parallel with the series circuit.
A resonating output signal is obtained from an input/output terminal p4, and a direct-current control voltage from a control terminal p5 is divided as needed and applied to each connection of the variable-capacity diodes 12 and 13. In this way, by connecting the variable-capacity diodes 12 and 13 in a multi-stage series structure, stable resonance characteristics can be ensured, even if the resonating signal obtained from the input/output terminal p4 is high in voltage.
An alternative to the use of variable-capacity diodes (4, 5, 12 and 13 above) for obtaining desired filter characteristics is disclosed by, for example, Japanese Unexamined Patent Publication Nos. 2-302017/1990, 62-259417/1987, 62-281319/1987, and 63-128618/1988. This is a method in which capacitance is changed by the use of voltage-controlled variable-capacity capacitors.
FIG. 28 is a cross-sectional diagram schematically showing the structure of a voltage-controlled variable-capacity capacitor 21 according to Japanese Unexamined Patent Publication No. 2-302017/1990. This voltage-controlled variable-capacity capacitor 21 is structured so that, between a pair of parallel plate capacitive electrodes 22 and 23, a plurality of bias field applying electrodes 24 and oppositely charged bias field applying electrodes 25 alternate with each other, with ferroelectric ceramic material lying between these electrodes.
By connecting a bias power source 26 between the bias field applying electrodes 24 and the bias field applying electrodes 25 and changing the direct-current voltage outputted by the bias power source 26, the electric field applied to the ferroelectric ceramic material is changed, thereby causing the dielectric constant to change. Thus the capacitance of the ferroelectric ceramic material is changed. Accordingly, in the voltage-controlled variable-capacity capacitor 21, variable capacitance can be produced within the ceramic substrate itself.
When structuring a high-frequency circuit module using the voltage-controlled variable-passband filter 1 or the voltage-controlled variable-capacity capacitor 21, in the interests of small size, it is desirable to form the circuit pattern within a multi-layer substrate. However, since actual component mounting and other steps of the assembly process tend to create unevenness, it becomes necessary to prepare in advance a pattern for adjustment purposes, and to make adjustments by trimming the adjustment pattern while confirming the circuit characteristics, until the desired characteristics are obtained.
In other words, as shown in FIG. 29, when mounting and soldering of components and other operations for assembly of a module have been completed in Step q1, the module is inspected in Step q2. Trimming adjustment is made in Step q3 on the basis of the inspection results, and then a further inspection in Step q4 and further trimming adjustment in Step q3 are repeated until the desired characteristics are obtained, after which the module is shipped in Step q5.
Further, in structures which use variable-capacity diodes like those mentioned above (4 and 5 in FIG. 26 and 12 and 13 in FIG. 27), semiconductor materials such as Si, GaAs, and Ge are used for these variable-capacity diodes 4, 5 and 12, 13. Accordingly, it is not possible to integrally provide these variable-capacity diodes 4, 5 and 12, 13, and the remainder of the circuit within the ceramic substrate. Thus, they must be attached externally after the high-frequency filter circuit substrate is formed. Accordingly, these structures have the drawback that the number of components and assembly steps is increased.
Further, the characteristics of these variable-capacity diodes 4, 5 and 12, 13 are influenced by the high-frequency signals which are to be handled, but when the variable-capacity diodes 12 and 13 are connected in a multistage series as in the resonating circuit 11, this influence can be reduced.
However, since the required control voltage increases in proportion to the number of series stages of the diodes 12 and 13, thereby burdening the control voltage source, and with battery-driven portable devices there is the drawback that a booster circuit must be used to boost the low power source voltage to a voltage corresponding to the required control voltage.
In the voltage-controlled variable-capacity capacitor 21, which is made of ferroelectric ceramic material, the bias field applying electrodes 24 and 25 are provided between the two terminal electrodes 22 and 23; however, although the dielectric constant of the ferroelectric material between the bias field applying electrodes 24a and 25a (the shaded area in FIG. 30 (a)) is changed, that of the area outside the bias field applying electrodes 24a and 25a is not changed.
Accordingly, the equivalent circuit for this structure, as shown in FIG. 30 (b), is one in which a variable-capacity capacitor 29 with relatively high capacitance is connected in series between two other fixed-capacitance capacitors 27 and 28 with relatively low capacitance. Accordingly, given the characteristics of serial connection of capacitors, the influence of the relatively low-capacitance terminal capacitors 27 and 28 is great, and even a great change in the capacitance of the relatively high-capacitance capacitor 29 will not greatly change the total composite capacitance. Thus the problem remains that a great change in bias voltage is necessary to greatly change the composite capacitance.
Another problem with the conventional art is that, when trimming is used to adjust the characteristics of the high-frequency circuit module, excessive trimming cannot be restored, and since adjustment becomes impossible, the yield is reduced.